Those are all good suggestions.

Since you are limited to a 13" or possibly 12" prop length, you could also use a 3-blade prop which gives you an effective added inch. (i.e. 12" 3-blade = 13" 2-blade)

Also, using two 3-cell packs in series can alleviate your charger requirements from a 5-cell pack. Using two 3s packs in series gives you a 6s voltage equal to 20 cells NiCd. This is within the AXI 4120/18 specifications. Use a Lithium pack that can deliver 40amps (or more) continuous like the [link=http://www.hobby-lobby.com/thunderpower.htm]ThunderPower 3s2p 4200mAh[/link] pack or the [link=http://www.hobby-lobby.com/kokam.htm]Kokam 3-cell 3.2AH[/link] pack. On a plane this size, the shape of the pack may be your determining factor.

Good luck!